I don't have the slightest clue who she could be. My dad, myself, my boyfriend, and my nephew will all be cosplaying from different video games, but she doesn't have to cosplay anyone from a video game. It also doesn't have to be very elaborate, since we've only got a month to put everything together and I'm stuck making everyone's cosplays.
Last year she let me make her up as a zombie, but I doubt she'd want to do that again.

If she enjoys the Mario series, she could get a pair of blue overalls, appropriate colored shirt, and a hat to be either a female Mario or Luigi (whichever is her favorite). She'd just have to decorate the hat with the symbol.

If she likes any of the games the rest of the group is dressing up from, it might also be fun to try to come up with a costume to include her with someone else in the group. (Assuming she also likes those games, of course! )